1. Field of the Invention
The present invention relates to a device for transporting a first light image by means of an optical cable and combining it with a second light image in order to permit combined observation of both images. The use of the invention is more particularly contemplated in the field of helmet sight visors for viewing a firing reticle or a synthetic image superimposed on a view of the surrounding landscape.
2. Description of the Prior Art
The term "optical cable" is understood to designate an ordered bundle of optical fibers or in other words a close-packed assembly of light-conducting fibers so arranged that the ends corresponding to a given fiber are located in homologous positions at x and at y on the two flat end faces of the bundle. The homologous arrangement of the fibers at each end permits image transport.
The result achieved by the basic design of an ordered bundle is that, if an image is formed by an optical lens on a flat end face, the light energy is transmitted by the fibers at an array of points and is again ordered in the same manner at the opposite end, whereupon the light image is re-formed, with due allowance for transmission losses. When the exit image is employed for visual display, the observer usually views the image through an eyepiece or magnifying lens in order to benefit from the enlargement thus produced since the cable has relatively small dimensions. In another application, the exit image is transported by the exit optical system to the light-receiving target of a television camera tube or to the matrix array of a charge-coupled device (CCD).
These devices are subject to disadvantages which arise from re-grouping of the cable fibers at x and y since this produces a raster effect. In order to overcome this drawback as well as the difficulties arising from fiber cuts, a known practice disclosed in French patent No. FR-A-2 504 758 (or European patent No. EP-A-0 064 899) consists of making use of light-image transport by optical cable in which means are provided for eliminating the raster effect. These means consist of a first deflecting prism which is placed upstream of the entrance objective and which produces dispersion of radiation as a function of the different wavelengths. The result thereby achieved is that the radiation corresponding to one image point is focused by the entrance objective on a number of fibers of the entrance face of the optical cable. Similarly, the exit face of the cable is followed by an exit lens in order to permit viewing of the image and said exit lens is in turn followed by a second prismatic element for inversely combining the exit radiations and re-forming the image.